Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process in which the first step involves alkylation of benzene with propylene in the presence of an acidic catalyst to produce cumene. The second step is oxidation, preferably aerobic oxidation, of the cumene to the corresponding cumene hydroperoxide. The third step is the cleavage of the cumene hydroperoxide into equimolar amounts of phenol and acetone, a co-product.
It is also known that phenol and cyclohexanone can be co-produced by a variation of the Hock process in which cyclohexylbenzene is oxidized to obtain cyclohexylbenzene hydroperoxide and the hydroperoxide is decomposed in the presence of an acid catalyst to the desired phenol and cyclohexanone. Although various methods are available for the production of cyclohexylbenzene, a preferred route is disclosed in U.S. Pat. No. 6,037,513, which discloses that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt and mixtures thereof. The '513 patent also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide, which is then decomposed to the desired phenol and cyclohexanone co-product.
There are, however, a number of problems associated with producing phenol via cyclohexylbenzene rather than the cumene-based Hock process. Firstly, oxidation of cyclohexylbenzene to cyclohexylbenzene hydroperoxide is much more difficult than oxidation of cumene and requires elevated temperatures and the use of a catalyst, generally a cyclic imide, such as N-hydroxyphthalimide (NHPI), to achieve acceptable rates of conversion. Not only are cyclic imide catalysts expensive but also, unless removed from the oxidation products prior to the cleavage step, they may cause problems in downstream separation processes and affect the quality of the final products. There is therefore strong incentive to separate the cyclic imide catalyst from the oxidation effluent. In addition, the cleavage chemistry for cyclohexylbenzene hydroperoxide is more complicated than that for cumene hydroperoxide, particularly since more routes for by-product formation exist with cyclohexylbenzene hydroperoxide cleavage. Moreover, cyclohexanone is more prone to acid-catalyzed aldol condensation reactions than acetone so that significant yield loss is possible unless the cyclohexylbenzene hydroperoxide cleavage step is closely controlled.
In the conventional cumene-based Hock process, the cleavage catalyst is normally sulfuric acid. However, even for the cleavage of cumene hydroperoxide, there are significant disadvantages of using sulfuric acid as the catalyst in that 1) sulfuric acid is corrosive, especially in the presence of water, requiring expensive materials for reactor construction; 2) sulfuric acid needs to be neutralized before product separation and distillation, which requires additional chemicals such as phenate, caustics, or organic amines; and 3) the salt generated from neutralization requires separation and disposal and the waste water needs to be treated. Therefore, there are strong incentives to replace sulfuric acid with a heterogeneous cleavage catalyst that eliminates these drawbacks.
The patent and academic literature is replete with suggestions for replacing sulfuric acid in the cleavage of cumene hydroperoxide. For example, U.S. Pat. No. 4,490,565 discloses that zeolite beta is an effective replacement for sulfuric acid in the cleavage of cumene hydroperoxide and indicates that the yields, conversions and selectivities are generally superior to those produced by the use of the large pore zeolites X and Y. In U.S. Pat. No. 4,490,566, similar improvements over the large pore zeolites X and Y are reported with intermediate pore size zeolites, such as ZSM-5. In contrast, in an article entitled “Efficient Cleavage of Cumene Hydroperoxide over HUSY zeolites: The role of Bronsted activity”, Applied Catalysis A: General, 336 (2008), pages 29-34, Koltonov et al. report that cumene hydroperoxide readily undergoes decomposition over HUSY zeolites of high (15 to 40) Si/Al ratio with good selectivity to phenol and acetone and with efficiency even comparable to that of sulfuric acid. Despite, or possibly because of, these varying recommendations, most commercial processes for the cleavage of cumene hydroperoxide continue to use sulfuric acid as the catalyst.
Less interest has been focused on the cleavage of cyclohexylbenzene hydroperoxide, although International Patent Publication No. WO2011/001244 discloses that cyclohexylbenzene hydroperoxide can be converted to phenol and cyclohexanone in the presence of a variety of homogeneous or heterogeneous acid catalysts selected from Brønsted acids and Lewis acids. Suitable homogeneous catalysts are said to include protic acids selected from sulfuric acid, phosphoric acid, hydrochloric acid, and p-toluenesulfonic acid. Solid Brønsted acids such as Amberlyst and Lewis acids selected from ferric chloride, zinc chloride, and boron trifluoride are also disclosed. In addition, suitable heterogeneous acids are said to include zeolite beta, zeolite Y, zeolite X, ZSM-5, ZSM-12, and mordenite. However, in the process of WO2011/001244, the catalyst used in the oxidation step is removed from the oxidation effluent before the cyclohexylbenzene hydroperoxide is fed to the cleavage step.
According to the present invention, it has now been found that acidic molecular sieves, such as FAU type zeolites, are not only effective catalysts for the cleavage of cyclohexylbenzene hydroperoxide, they are also effective adsorbents for the cyclic imide catalysts employed in the oxidation reaction. Thus, by using an acidic molecular sieve as the cleavage catalyst, it is possible to remove the cyclic imide catalyst from the oxidation effluent simultaneously with the conversion of the cyclohexylbenzene hydroperoxide to phenol and cyclohexanone. In this way the need for a separate step of removing the cyclic imide catalyst from the oxidation effluent can be obviated. Once acidic molecular sieve has become saturated with the cyclic imide, the adsorption/cleavage process can be temporarily suspended to allow the cyclic imide to be desorbed from the molecular sieve for recycle to the oxidation step. After removal the cyclic imide, the regenerated molecular sieve can be returned to adsorption/cleavage duty.